1. Field of the Invention
The present invention relates generally to a braking system, and more particularly to techniques for improving braking performance of a motor vehicle's braking system by optimizing the distribution of braking forces between the front and rear wheels of the vehicle.
2. Discussion of the Related Art
FIG. 1 illustrates the braking forces of a motor vehicle's front and rear wheels as an upwardly convex curve where the front and rear wheels begin to lock simultaneously on a road surface. In FIG. 1 the braking force of the rear wheel is plotted on the vertical axis while the braking force of the front wheel is plotted on the horizontal axis. This curve, referred to as an "ideal distribution curve," represents an ideal distribution of the braking forces applied to the front and rear wheels. To improve the braking capacity or performance of the braking system, the specifications of the braking system should be optimized so that an actual distribution curve, representing the actual distribution of the front and rear wheel braking forces, is as close as possible to the ideal distribution curve. The specifications of the braking system include, for example, diameters of the front and rear wheel brake cylinders, effective radii of the front and rear disc brake rotors, and inside diameters of the front and rear wheel brake drums.
While the ideal distribution is represented by a curve as described above, the actual distribution of the front and rear wheel braking forces, as established by the basic arrangement of a braking system, is represented by a straight line, as indicated in FIG. 1. The basic arrangement is not provided with a load-sensing proportioning valve (LSP valve). It should be noted that the rear wheel braking force is not constant, but increases as the amount of load acting on the vehicle increases with respect to the minimum load (i.e., a load acting on the vehicle during a minimum-load run of the vehicle). A "minimum-load run" means a run of the vehicle with only the driver (without any passengers, in the case of a passenger car, or without any cargo, luggage or load, in the case of an industrial vehicle). FIG. 1 further illustrates that the ideal distribution curves during a minimum-load run of the vehicle differs from that during a full-load run of the vehicle. The "full-load run" means a run of the vehicle with the nominal number of passengers (including the driver), in the case of the passenger car, or the nominal maximum load, in the case of the industrial vehicle.
Further, upon braking of the vehicle the braking system is generally designed to avoid locking of the rear wheels prior to locking of the front wheels to prevent the vehicle from losing control in the running direction. The braking system is also designed to prevent locking of the rear wheels prior to front wheels during the minimum-load run where the load acting on the rear wheels is the smallest, and corresponding to the highest locking tendency of the rear wheels. Described in greater detail, the braking system is usually adapted to minimize a deviation of the actual distribution of the front and rear wheel braking forces (i.e., a deviation of the basic distribution line as determined by the specifications of the most basic braking arrangement indicated above) from the ideal distribution curve in a direction that causes an increase in the rear wheel braking force.
In practice, however, it is difficult to design a braking system with an actual distribution line of the front and rear wheel braking forces that is sufficiently close to the ideal distribution line. As shown in FIG. 1, the actual distribution line has a larger amount of deviation from the ideal distribution curve during the full-load run of the vehicle than during the minimum-load run. The deviation is attributable to the difference between the ideal and basic rear wheel braking force values. In other words, designing braking systems to accurately follow the ideal distribution curve is limited since the actual basic distribution is generally represented by a straight line and the ideal distribution curve varies as a function of vehicle load.
While the basic arrangement of the braking system has the drawback as described above, an improved arrangement also exists where a proportioning valve is disposed between the hydraulic pressure source and a rear wheel brake cylinder. This arrangement results in an actual distribution line which is closer to the ideal distribution curve. As indicated in FIG. 1, the actual distribution lines of the proportioning valve (load-sensing proportioning or LSP valve) are bent straight lines which are closer to the ideal distribution curve than the basic distribution lines. As disclosed in laid-open Publication No. 2-130870 (published in 1990) of unexamined Japanese Utility Model Application, the proportioning valve is a pressure reducing valve that is adapted to reduce the hydraulic pressure generated by the hydraulic pressure source at a predetermined ratio and apply the reduced hydraulic pressure as the braking pressure to the rear wheel brake cylinder after the generated hydraulic pressure has exceeded a predetermined threshold level. Until the generated hydraulic pressure reaches the threshold level (indicated by dots in FIG. 1 at the points of bending of the actual distribution lines of the proportioning valve), the proportioning valve does not function as the pressure reducing valve, and the hydraulic pressure generated by the pressure source is applied to the rear wheel brake cylinder.
In industrial vehicles, such as trucks where the load acting on the rear wheels varies considerably as the amount of cargo varies, the braking capacity or performance is insufficient when the load on the rear wheels is relatively large and the threshold level indicated above is fixed (i.e., if the level of the generated hydraulic pressure at which the proportioning valves begins to function as the pressure reducing valve is fixed). In view of this drawback, the braking system for such industrial vehicles is equipped with a load-sensing proportioning valve also known in the art. In the load-sensing proportioning valve (generally referred to as "LSP valve", or "LSPV"), the threshold level which corresponds to the point of bending of the distribution line of the valve varies as a function of in the amount of load on the vehicle. There are two types of load-sensing proportioning valve: linkage and ball. The linkage LSPV utilizes the fact that the amount of relative displacement between portions of a sprung member and an unsprung member the rear wheel assembly increases with the load that acts on the rear wheels. Thus, the linkage LSPV is adapted to detect the vehicle load in the form of the relative displacement amount of the sprung and unsprung members. The ball LSPV utilizes the fact that the rear portion of the vehicle body is raised in relation to the front portion as the load on the rear wheels decreases. The ball LSPV uses a ball adapted to roll on an inclined surface wherein the inclination angle changes with the inclination angle of the vehicle body, so that the ball is seated on a valve seat as a result of rolling. In the ball LSPV, the difficulty of rolling of the ball on the inclined surface indicates vehicle load.
However, the degree of approximation of the distribution line of the load-sensing proportioning valve to the ideal distribution curve is limited. That is, it has been difficult to sufficiently solve the undesirable tendency that the actual distribution line of the load-sensing proportioning valve deviates from the ideal distribution curve, in the direction that causes the rear wheel braking force to be smaller than the ideal value. This is particularly problematic when the vehicle is in the full-load run. As illustrated in FIG. 1, the hatched area is an area of deviation of the actual rear wheel braking force from the ideal value. Therefore, during the full-load run of the vehicle, the actual braking forces applied to the rear wheels are considerably lower than the ideal value, or cannot be increased to the optimum value. Thus, the use of load-sensing proportioning valves still suffers from insufficient rear wheel braking forces, although rear wheel locking is prevented.
The above-identified problem of increasing the rear wheel braking force to the ideal or optimum value during the full-load vehicle run also exists in known anti-lock braking systems adapted to control wheel braking pressures. The anti-lock control of braking forces will be described below in detail.
Braking systems are classified into two types: independent front-rear braking force control and diagonal or X-crossing. In the independent front-rear braking force control type, the first pressure application sub-system, including the front right and left wheel brakes, is independent of the second pressure application sub-system, including of the rear right and left wheel brakes. In the X-crossing type, the first pressure application sub-system, including the front left wheel brake and the rear right wheel brake, is independent of the second pressure application sub-system, including the front right wheel brake and the rear left wheel brake.
In an anti-lock braking system of the independent front-rear braking force control type, the front and rear wheel braking pressures are usually regulated independently of each other during anti-lock braking pressures control. In this case, the actual front-rear distribution of the braking forces is not bound by the basic distribution line determined by the specifications of the braking system, but can be changed with a high degree of freedom from the basic distribution line. Accordingly, the actual distribution line can be made sufficiently close to the ideal distribution curve. Therefore, the braking system of the independent front-rear braking force control type does not suffer from the above-identified problem that the rear braking forces cannot be increased sufficiently during the full-load run of the vehicle.
In an anti-lock braking system of the X-crossing type, several arrangements are available for anti-lock control of the braking forces. One example of such arrangements is illustrated in FIG. 2, wherein a normally-open master cylinder cut-off valve 306 is provided in a front brake cylinder passage 304 connecting a master cylinder 300 (hydraulic pressure source) and a front wheel brake cylinder 302, while a rear brake cylinder passage 308 is connected at one end thereto to a portion of the front brake cylinder passage 304 between the cut-off valve 306 and the front wheel brake cylinder 302. The rear brake cylinder passage is connected at the other end to a rear wheel brake cylinder 307. A normally-closed shut-off valve in the form of a pressure reducing valve 312 is provided in a reservoir passage 310, which is connected at one end thereto to the rear brake cylinder passage 308 and at the other end to a reservoir 316. The reservoir 316 which receives the brake fluid discharged from the wheel brake cylinders 302, 307 through the shut-off valve 312. A pump 318 is connected to the reservoir 316 to return the brake fluid to the master cylinder 300. According to this braking arrangement, the braking pressures in the front and rear wheel brake cylinders 302, 307 are increased by the pressure generated by the master cylinder 300.
The assignee of the present application proposed another braking arrangement of the X-crossing type, as shown in FIG. 3. Unlike the braking arrangement of FIG. 2, the present braking arrangement of FIG. 3 is adapted to increase the braking pressures in the front and rear wheel brake cylinders by operation of the pump 318. That is, the master cylinder cut-off valve 306 is held closed during an anti-lock control of the braking pressures and the pump 318 is connected to a portion of the front brake cylinder passage 304 which is downstream of the cut-off valve 306. Correspondingly, the pressurized fluid from the pump 318 is not returned to the master cylinder 300 but is returned to the wheel brake cylinders 302, 307, whereby the braking pressures in the wheel brake cylinders are increased by operation of the pump 318 during the anti-lock control of the braking system.
In either of the two arrangements of the anti-lock braking system of the X-crossing type, the braking pressures in the front and rear wheel brake cylinders 302, 307 cannot be regulated independently of each other, but are regulated such that the braking pressure in the front wheel brake cylinder 302 is equal to that in the rear wheel brake cylinder 307. Therefore, unlike the braking system of the independent front-rear braking force control type, the braking system of the X-crossing type is not capable of establishing the actual distribution line which is shifted from the basic distribution line in the direction that causes an increase in the braking pressure in the rear wheel brake cylinder during the anti-lock control. Thus, like the ordinary braking system incapable of effecting the anti-lock control of the braking forces, the braking system of the X-crossing type suffers from the problem of insufficient rear wheel braking force during the full-load run of the vehicle.
Further arrangements of the anti-lock braking system of the X-crossing type are illustrated in FIGS. 4 and 5. In the arrangement of FIG. 4, two 3-position valves 320 each having a pressure-increase position, a pressure-hold position and a pressure-decrease position are provided for the front and rear wheel brake cylinders 302, 307, respectively. In the arrangement of FIG. 5, a series connection of two shut-off valves 322, 324 is provided for each of the front and rear wheel brake cylinders 302, 307, in place of the 3-position valve 320 used in the arrangement of FIG. 4. Although these arrangements of FIGS. 4 and 5 permit the actual distribution of the front and rear wheel braking forces to be controlled without restriction by the basic distribution line, these arrangements suffer from the separate problem inevitably complicated construction, which corresponds to increased manufacturing cost.